The invention generally relates to a protective mechanism and an optical coupler for use in systems for detecting the presence of hydrocarbons during mining or drilling operations. In the prior art, special optical couplers using Sylgard along with optical coupling oil have been employed with prior support systems to couple light from a scintillation element into a light detector device. Such an optical coupler is disclosed in U.S. Pat. No. 6,465,788, which is incorporated herein by reference in its entirety. One drawback to this approach is that, under some extreme cases of high loads, uneven loads, or high vibration, oil used in the optical coupling may migrate out over time and result in degraded detector performance. Another drawback is that precision fabrication and/or assembly tolerances must be maintained to prevent loss of oil and degraded performance. Yet another drawback is that particulate contamination of the optical coupler can also cause loss of oil and degraded performance. Another example of an optical interface is disclosed in U.S. Pat. No. 6,222,192 to Sekela et al., the entire disclosure of which is incorporated herein be reference.
Optical couplers made from self-wetting type materials (e.g., Wacker) have also been used. A drawback to these concepts is that the self-wetting materials exhibit viscous behavior and tend to flow outward from the optical interface, allowing the optical interface retaining force to be lost, and thus resulting in degraded performance. Wacker is an example of a self-wetting, optically clear material that is used for optical coupling, and is sometimes the material of choice. The only materials otherwise suitable for use inside a hermetic housing that contains a sodium iodide crystal, which also is capable of withstanding substantial dynamic loading and stresses, are not optically clear, and or do not provide a consistent high quality optical interface. However, Wacker and other similar materials, cannot withstand substantial loading and/or will produce false scintillations under vibration due to movements. Previous efforts to use this material include attempting to limit longitudinal loading on the material but result in the crystal assembly moving longitudinally during high longitudinal vibration and/or result in failure to move to maintain optical coupling under large changes in temperature.
Nuclear detectors, such as gamma detectors, have been used in mining applications and oil drilling operations for many years. In particular, gamma detectors have been used to measure the radiation that emanates from the formations surrounding the mining or drilling equipment. Such gamma detectors operate by utilizing the differences between the natural radioactivity of the target formation and the natural radioactivity of the adjacent formations to determine the boundaries between these formations. In the case of mining potash, the most desirable material to be mined from the formation is the most radioactive, typically being surrounded by salt or lower grade mineral.
Gamma detectors are sensitive and must be protected from harsh environments to survive and to produce accurate, noise free signals. This protection must include protection from physical shock and stress, including force, vibration, and abrasion, encountered during solid mineral mining and oil drilling operations. However, the closer in proximity the gamma detector is to the mineral being mined or drilled, the greater is the shock, vibration and stress to which the detector is subjected.
The presence of armor, which is required to protect the detector, further limits the available space. An explosion-proof housing takes up even more of the available space, and often results in reducing the diameter of the photomultiplier tube. When light detecting devices of relatively low mass density are used in connection with scintillation elements having a relatively high mass density, a special means of support is needed to reduce rotation moments when under high vibration or high shock. Lower cost for providing protection for the detector is also needed.
Advances have been made in recent years that improve the survivability and performance of gamma detectors that are used in mining, drilling, and other harsh environments. Yet, there remains a need for further improvements. One area of need arises whenever large scintillation crystals are used in a harsh environment such as mining. Long term wear and damage to the support system from continual high shocks can occur due to the larger mass of the scintillation element. Shock isolation must be done with sufficient care to not damage the interface between the crystal and the light collecting element. Another area of need is for a support system that can be designed with less engineering and analytical expertise, so that components can be fabricated with more ease and at a lesser cost.
A support system must be very effective in protecting the detector from the harsh vibrations and shock, but must also do so while consuming a small amount of space. Similarly, in mining operations, the outer portions of the detector and the armor must provide a high level of shielding from unwanted radiation and must protect the detector from impact and abrasion, all with a minimal use of space.
Radial springs, although effective in other applications, have not been utilized in subject applications, because, for example, radial springs have been found to be difficult to install, particularly for large scintillation elements and especially for large detectors. Also, the selection of the width, thickness, and design of radial springs in the applicable spaces of gamma detectors has been found to be complex, thus discouraging their use in some instances.
In the prior art, detectors have been protected by a plurality of springs which extended along the axial length of the detector or its scintillation element. An example of such a support system is a flexible dynamic housing, as disclosed in U.S. Pat. Nos. 6,452,163 and 6,781,130, which are incorporated by reference herein in their entireties. One drawback of such systems is that the springs extend along the axial length of the scintillation element and as such can block radiation from reaching the scintillation element, which is particularly important where rapid motion of the cutter necessitates obtaining the maximum possible gamma count rate. Moreover, the springs of the flexible housing have to be custom made for this specific industrial application. Also the annular gap that exists between the scintillation element and its rigid housing is not always uniform, such as because of dimensions of tolerance. This may complicate the installation or sizing of the system.
Flexible dynamic housings and flexible sleeves helped to solve some problems. One very important characteristic of these supports is the reliance upon friction to hold the scintillation element, photomultiplier tube, and other elements in position during high vibration, while allowing for thermal expansion and shock. This reliance upon friction, instead of elastomeric materials reduces resonances, providing a dynamic transmissibility of near unity through most frequencies of concern and then provides effective dynamic damping once the friction is overcome. However, complexities in their design and fabrication resulted in higher cost than desired, requiring special engineering, processes, and specialized fabrication procedures. Experience has shown that there is a need to improve upon the advantages of using metallic supports and the use of friction to improve the ability to withstand high vibration and high shock as when used on rotary cutters. Improvement is needed to reduce design and fabrication complexity, and thereby reduce cost.
Another support mechanism for a detector is disclosed in pending application Ser. No. 10/270,148, which is incorporated herein by reference in its entirety. This type of support mechanism is a flexible support sleeve which extends along the length of the detector or scintillation element, and suffers from the same drawbacks discussed above with respect to the springs. Furthermore, very high shock conditions, particularly for larger crystals, can over stress flexible sleeves at the bends of such sleeves, causing the contact pressure to be reduce and thereby having insufficient friction remaining for good support.
There remains a need for an optical coupling system that is less sensitive to fabrication/assembly tolerances, high/uneven loads, and high vibrations. There is also a need for a simplified, lower cost structure and method for supporting instrumentation packages and sensors, such as gamma detectors. A means for supporting sensitive elements, which have substantially a cylindrical shape, is needed to work in cooperation with other suitably chosen support elements. A more suitable method of supporting sensitive elements so as to produce less compression of optical reflecting material is also needed.
Through the years, even at the present time, use has been and is being made of elastomeric or rubber materials in an effort to protect scintillation elements, photomultipliers, electronics, and assemblies of these items while being used in harsh environments. Although elastomers have proven to useful for cushioning high shock, high vibration combined with high shock has proven to be very challenging for protecting fragile elements such as sodium iodide crystals or cesium iodide. If wide temperature excursions are also involved, the problem is even more challenging. There are fundamental reasons why this is the case. For one thing, these materials, which are much softer than metals, tend to produce a low resonant frequency. This contributes to higher forces being placed upon the objects being protected. Resonating at lower frequencies results in greater displacements of the elements and increases the probability of spontaneous noise generation and/or damage.
In an effort to reduce these effects, one may compress the materials around the object being protected so that there is less room for it to move. If subjected to large temperature changes as is experienced during drilling into the earth or on hot machinery during cutting, the scintillation element expands toward the metallic shield, thus placing excessively high pressure on the element. This is made worse by the expansion of the elastomeric or silicone rubber material, which usually has a very large coefficient of expansion as compared to other parts of the support system. Not only can these high forces damage the elements being supported but they can cause interfacing element such as a scintillation element to be pulled away from the photomultiplier. Trying to overcome such a separation by placing more force onto the interface by using larger springs to force the two together has sometimes been shown to break the face of the photomultiplier tube, or the coupling, or the scintillation element. Yet, attempts to overcome this problem by mechanically limiting the forces placed on the interface tends to recreate the problem trying to be solved. Reduction of the restraining forces allows the interfacing elements to resonate in their longitudinal direction.
Added to the above is the fact that the internal damping characteristics of elastomeric materials or silicone rubber are poor compared to that of sliding friction. The result is that when resonance is made possible by the geometric considerations described above, the magnitude of the resonance is greater than it would be if sliding friction were more prevalent. Thus, the limits of applicability for such materials do not satisfy the needs of the industry.